Probably the best option is to buy a VRLA (Valve Regulated Lead Acid)battery.There are many options on output, amp hrs and so on.They are the grey batteries that are used in race cars, bserver back up power supplies and house alarms.Best bang for you buck!

It looks to have multiple batteries, but difficult to say if they are on series or parallel. Anyway, the announced mAh rating is for sure theoretical max for capacity of each battery summed. And it has an inverter to provide suitable voltages.

My concern would be how good electricity that inverter provides, could cause some noise on monitor or audio and in worst case break something. The build quality and safety are typical at this price point, not very good. Hot glued parts etc. Maybe some safety circuits maybe not.

Also it is unclear how the voltage is adjusted. great care should be taken not to provide more than 12 V.